In a gasification furnace which produces combustible gas by thermally decomposing a pulverized raw material composed of a carbonaceous solid, ash of the raw material is melted and deposited on a bottom portion of the gasification furnace as slag. In order to receive the slag, a water tank with cooling water retained therein is provided, and the slag is rapidly cooled by the cooling water, thereby being crushed and solidified. The slag accumulated in the water tank of the gasification furnace in a state of being solidified in this manner is discharged out of the gasification furnace by way of a lock hopper. Since the slag has a large difference in density from that of water that is the cooling water, in the past, when moving the slag from the water tank to the lock hopper, gravity has been used as a driving force. For example, in PTL 1, a slag discharge system in which a lock hopper is disposed below a gasification furnace is disclosed.
A slag discharge system 100 like this will be described using FIG. 7. A lock hopper inlet valve 105 is opened in a state where a lock hopper outlet valve 103 below a lock hopper 101 is closed, whereby slag settles out of a gasification furnace 107 from the inside of a water tank 109 provided at a lower portion of the gasification furnace 107. Subsequently, the slag accumulated in the lock hopper 101 is discharged to a slag relay tank 111 by opening the lock hopper outlet valve 103 in a state where the lock hopper inlet valve 105 is closed. The slag is transferred from the slag relay tank 111 to a slag slurry tank 115 by a screw conveyor 113. After the slag is slurried in the slag slurry tank 115, the slurried slag is pumped by using a slurry transfer pump 117, thereby reaching a dewatering tank 121 by way of a distributor 119. Thereafter, the slag is transferred to another treatment system by using a vehicle or the like.